Kinetics and Catalysis最新文献

The article is devoted to the analysis of possible spatiotemporal kinetic structures that can arise during catalytic oxidation reactions on metal surfaces at atmospheric pressure. The catalytic oscillatory reaction in a flow reactor is modeled using a 1D system of equations of the reaction–diffusion–convection type. The STM type oscillatory reaction model of catalytic oxidation is used as a kinetic model. The obtained results of mathematical modeling show the decisive influence of an axial mixing in the reactor on the development of spatiotemporal structures. It is also shown that, depending on the ratio of adsorption rates of reacting species, three different isothermal spatiotemporal structures can arise, namely a spatially inhomogeneous stationary state, regular and aperiodic “breathing structures”.

The number of observations that demonstrate a significant effect of the size of droplets on the kinetics of chemical processes has increased with the expansion of the scope of applications of spray technology. The equations linking the concentrations of reagents, the volume of droplets, the initial composition of a solution, the composition of the gas medium, and the speed of processes are formulated within the framework of formal chemical kinetics. Using second order reactions (coupling, exchange, condensation, polymerization, and polycondensation reactions) as examples, it is shown that size kinetic effects occur when chemical processes are accompanied by changes in the droplet sizes in equilibrium with the gas medium. The results of computer simulation of condensation and polycondensation reactions are given, which reproduce size effects. Kinetic curves obtained by simulation of the polycondensation process are compared with experimental data.

The influence of platinum additives on the properties of rhodium catalysts in the processes of steam reforming and autothermal reforming of diesel fuel was investigated. It was found that Rh/CZF was more active compared to the bimetallic sample Rh–Pt/CZF: the degree of fuel conversion in its presence was higher, and the concentration of reaction by-products was lower. The proposed two-zone Pt/CZF + Rh/CZF structured honeycomb catalyst demonstrated stable performance and high activity in the autothermal reforming of commercial diesel fuel. However, the presence of platinum in the frontal zone of the catalyst reduced its resistance to coking compared to the rhodium-containing sample. The results obtained are of practical significance in the development of efficient systems for the conversion of heavy hydrocarbons into synthesis gas.

Efficient heterogeneous catalysts for chemoselective hydrogenation of terminal and disubstituted alkynes and alkynols to monoenes based on Pd–P particles have been proposed. The influence of a zeolite support (Na-ZSM-5, MSM-41) and a phosphorus modifier on the properties of palladium catalysts in the semihydrogenation of acetylenic compounds is considered. Promotion with phosphorus increases the activity of palladium catalysts in hydrogenation of various acetylenic compounds 2.5- to 30-fold without reducing the selectivity for monoenes. The high selectivity for monoenes is determined by both thermodynamic and kinetic factors. The possibility of changing the ratio of the rates of hydrogenation of triple and double bonds by varying the nature of the solvent and the structural order of the catalyst particles was demonstrated using hydrogenation of acetylenic alcohols as an example.

Results of studying the catalytic properties of systems based on complexes with [Pd(Cp)(L)_n]_m[BF₄]_m (where Cp = η⁵-C₅H₅; n = 2, m = 1: L = tris(ortho-methoxyphenyl)phosphine, triphenylphosphine, tris(2-furyl)phosphine (TFP); n = 1, m = 1: L = 1,1'-bis(diphenylphosphino)ferrocene, 1,3-bis(diphenylphosphino)propane, 1,4-bis(diphenylphosphino)butane, 1,5-bis(diphenylphosphino)pentane; n = 1, m = 2 or 3: L = 1,6-bis(diphenylphosphino)hexane) in the addition homo- and copolymerization of norbornene (NB) and NB derivatives have been described. It has been found that these complexes can be activated with Lewis acids (BF₃⋅OEt₂ or AlCl₃). The productivity of the [Pd(Cp)(PPh₃)₂][BF₄]/BF₃⋅OEt₂ catalyst system in NB polymerization can achieve 188 800 mol_NB ({text{mol}}_{{{text{Pd}}}}^{{ - 1}}). The homopolymerization of 5-methoxycarbonylnorbornene and the copolymerization of NB with 5-methoxycarbonylnorbornene or 5-phenylnorbornene in the presence of BF₃⋅OEt₂ and [Pd(Cp)(L)₂][BF₄] (L = PPh₃ or TFP) has been studied. A hypothesis for the formation of the catalyst via the intramolecular rearrangement of the η⁵-Cp ligand into the η¹-Cp form upon interaction with a Lewis acid has been proposed. The structure of the [Pd(Cp)(TFP)₂]BF₄ complex (I) has been determined by X-ray diffraction (XRD) analysis. In the crystal structure of complex I, the coordination sphere of palladium is characterized by a slight distortion of the square planar geometry of the central atom, while the cyclopentadiyl moiety is in an eclipsed conformation. Based on XRD data, the steric hindrance of the TFP ligand has been determined (cone angle is 149°).

It has been shown in tests that N-heteroaryl-substituted α-diphenylphosphinoglycines, namely, N-(pyrazin-2-yl) α-diphenylphosphinoglycine, N-(pyridin-2-yl) α-diphenylphosphinoglycine, and N‑(pyrimidin-2-yl) α-diphenylphosphinoglycine, which are synthesized by the three-component condensation of diphenylphosphine, the respective primary amine, and glyoxylic acid monohydrate, being combined with Ni(COD)₂, where COD is cyclooctadiene-1,5, are capable of generating active forms of catalysts for the selective homogeneous dimerization and trimerization of ethylene to form butene-1 and hexene-1 as the main products. The studied organonichel catalyst systems provide a yield of short-chain (C₄–C₆) olefins at a level of 90% with a linear α-olefin selectivity of 97%. Studies of the effect of temperature on the homogeneous oligomerization of ethylene using the synthesized compounds have revealed that the process run at an optimum temperature of 80–105°C and an optimum ethylene pressure of 20–35 atm provides the highest butene-1 and hexene-1 selectivity. Under these conditions, the butene selectivity is recorded at a level of 71.4–72.6% (butene-1 selectivity of 69.3–71.1%), while the hexene selectivity is 20.6–21.2% (hexene-1 selectivity of 19.2–19.5%). The optimum duration of the oligomerization process at a temperature of 105°C is 1.5 h. The butene-1 and hexene-1 formation rate is 168.1 and 47.3 g_olig ({text{g}}_{{{text{Ni}}}}^{{ - 1}}) h^–1, respectively.

In this work, we used X-ray photoelectron spectroscopy (XPS) to perform a comparative study of the interaction of NO₂ with two samples of highly oriented pyrolytic graphite (HOPG), on the surfaces of which rhodium was preliminarily deposited by evaporation in a vacuum, at room temperature and a pressure of 10^–5 mbar. Before metal deposition, one of the HOPG samples was annealed in a vacuum at 600°C, and the other was bombarded with argon ions followed by exposure to air at room temperature for 1 h in order to introduce strongly bound oxygen atoms into the surface composition. After the deposition of rhodium onto the two HOPG samples, two model catalysts designated as Rh/C and Rh/C(A)–O were prepared. It was found that the interaction of NO₂ with Rh/C led to the oxidation of graphite with the destruction of the surface layer. The Rh particles remained in a metallic state, but they were introduced into the near-surface layer of the carbon support. On the contrary, when the Rh/C(A)–O sample was treated with NO₂, the deposited rhodium was partially converted into Rh₂O₃, while the graphite was oxidized to an insignificant degree and retained its original structure. The role of surface oxygen in the stabilization of graphite with respect to oxidation in NO₂ was discussed.

A new catalytic system for reductive С(sp²)–C(sp³) cross-electrophile coupling was designed: Pt^II iodide complexes in an acetone solution of NaI catalyze the coupling of methyl iodide with vinyl iodide to form propylene. Simultaneously, a small amount of 1,3-butadiene, the product of C(sp²)–C(sp²) coupling, is released. The total yield of the products with respect to the reacted vinyl iodide is almost quantitative. In a large excess of CH₃I, the C₂H₃I consumption is described by the pseudo-first-order kinetics. The cross-coupling occurs as the following sequence of steps: oxidative addition of CH₃I to Pt^II iodide complexes to form a methyl Pt^IV complex → reduction of the methyl Pt^IV complex with I^– to form the corresponding Pt^II derivative → oxidative addition of C₂H₃I to the Pt^II derivative → reductive elimination of organyl ligands from the intermediate methyl vinyl Pt^IV complex.

Pyrolysis of high-density polyethylene in the presence of aluminosilicate materials containing nickel oxide has been studied. The catalytic pyrolysis of plastics makes it possible to convert polymers into chemical compounds, which can later be used as additional sources of fuels and raw materials for chemical industry or polymer production. The physicochemical parameters of the materials containing nickel oxide were determined by Fourier-transform IR spectroscopy, X-ray diffraction analysis, N₂ physical adsorption method, thermogravimetric analysis, and pyrolytic gas chromatography. The chemical composition of polyethylene pyrolysis products was found to depend on the type of support used and the presence of nickel oxide in it. The presence of nickel oxide in the studied aluminosilicates increases the Lewis acidity and the content of aromatic compounds in the pyrolysis products. The activation energy of polyethylene pyrolysis in the presence of MSM-41 containing nickel oxide was calculated from the experimental data.

The physicochemical and catalytic (CO₂ hydrogenation) characteristics of Mo-containing catalysts were studied. The catalysts containing 8 and 15 wt % Mo oxide were prepared by impregnation of γ‑Al₂O₃ with ammonium paramolybdate, followed by drying and calcination at 500°C. The introduction of Mo oxide reduced the pore volume of the support and increased the average pore size, indicating that molybdenum oxide was distributed in the support pores. According to the X-ray diffraction analysis, the calcinated catalyst did not contain the crystalline MoO₃ phase. According to the Raman spectra, oxygen-containing formations were present on the catalyst surface, with Mo atoms tetrahedrally and octahedrally coordinated to the oxygen atoms. The impregnated MoO₃ was partially reduced with hydrogen during linear heating, starting from 320°C. The hydrogenation of CO₂ (gas composition, vol %: 30.7 CO₂, 68 H₂, the rest was N₂; 0.5 g sample) was studied under conditions of linear heating to 400°C. The main reaction was the reverse reaction of CO steam reforming. The contribution of methanation to CO₂ hydrogenation was small. An increase in the temperature and pressure had a positive effect on CO₂ conversion. When the pressure increased from 1 to 5 MPa, the CO content was approximately doubled. In the CO₂ hydrogenation, appreciable activity (although significantly lower compared to that of Mo-containing catalysts) was also exhibited by γ-Al₂O₃, preliminarily heated to 400°C in an H₂ flow. The activity of alumina also increased with pressure.